n8vi's AVR-tuned 2-meter double-conversion superhet project

Project on hold

If you see any glaring stupid in this design, please let me know. It's my first RF circuit. Hell, it's really my first analog circuit.

I've just updated the schematic image to clarify a lot of things and add a couple bits, so some of the text below may not make sense anymore. I'll clean that up soon.

It's a hodgepodge of bits of schematic I've stolen from other people's projects. The only bits really missing are the microcontroller and the power for the two ICs (which will probably need some filtering). Stay tuned for Eagle bits.

Remaining tasks

The first LO connection to the first mixer is completely wrong. FIND NEW SOURCE. Perhaps a redesign is in order? I like this design, it's cheaper.

Footprint the LM380 and the MCP4131.

Deal with each IC's power and bypass capacitor requirements.

The AD9859 needs 1.8v. FIND A GOOD SMT 1.8v REGULATOR.

I'll be using the SMT 5v regulators I already have for the AVR, it's max232, the digital pot, the SA602 and the SA605

The LM380 looks like it will do fine on an unregulated 12v supply. It won't work on 9v though, so 9v batteries are out.

Add test points to both the in phase and quadrature inputs to the FM demod mixer.

Perform schematic capture in eagle. Perhaps add more test points.

Order parts.

Reread every document I got stuff from and every datasheet seeking board layout clue.

Path through the radio

What follows is in approximate order of path through the radio (there are a few unions, such as the separate LO and IF inputs to the second mixer).

First LO (AD9859)

I dug around through all of Analog Devices' DDS chips, and this one is the cheapest that does the frequency I'm interested in. It also seems to be fairly conservative in it's current draw. It's datasheet is here. Information on amplitude shaping can be found on page 16, and information on the full scale control resistor, the resistor that sets output current, can be found on page 12. Note to self: don't forget, DAC output must be referenced to AVDD, not AGND (page 12 of datasheet).

Oh look, he's using a board with an HF filter on it, not a bare DDS. THIS PART NEEDS ATTENTION, IT'S WRONG. This may be a good time to redesign with a lower frequency cheaper DDS and an SA602-based frequency multiplier.

Front End Filter and First Mixer (SA602)

This is the bit between the antenna and pins 1 and 2 of the first mixer, the SA602. This whole piece of schematic was taken from an article called "Junk-Box Converters for 6 and 2 Meters" (log in to paid ARRL membership required prior to clicking this link) by Rod Kreuter, WA3ENK in the January 1997 issue of QST (someone else has made bits of schematic available here). Most of the rest of what's connected to the SA602 comes from this article, besides the first LO. The SA602's datasheet can be found here, and information on connecting an external LO is on page 4.

I don't get it - to me it looks like a parallel resonant circuit with a resonant frequency of 210.179MHz, with the antenna input fed into not quite the middle of the capacitance. Don't mind me, I've been spending a lot of time reading this years ARRL handbook ...

First IF, connection to second mixer

This was taken from a double conversion superhet of somewhat simpler design on page 9 of this pdf. This PDF is awesome and is where I got most of what I've learned in this design. It is linked again later down this page, probably multiple times.

Second LO (Xtal)

Almost everything connected to the SA605 was from this pdf, but I wanted a 10.245MHz LO so I could use a crystal I could find. I got the rest of the schematic from ... ???

Per AN1994, this appears to be a fundamental-mode colpitts oscillator.

Note to self: check browser history of 10/23.

I may have converted caps on a different frequency LO to reactances, changed frequency to 10.245 and converted back to capacitances. I may even have done this to a datasheet example.

Quadrature Tank

This is the bit that takes the signal we're inputting into one side of the FM demodulator, throws it 90° out of phase, then feeds it into the other side. As far as I understand, the SA605 then mixes these two signals to demodulate FM.

I learned a bit about IF cans here and their use in FM detection here.

Audio Section

Right, so the Audio level out of the SA605 is nominally 150 mV RMS across a 100k resistor (with a 15pF capacitor in parallel for FM de-emphasis) (page 4 for spec, page 6 for diagram). We'll pretend the capacitor isn't there since the cap's lowest reactance at any frequency we care about is over 3 Mohms at 3KHz. which in parallel adds up to 97.3 KOhms.

So, 150mV over 100k is ... 1.5 uA? So that's .2 uW? sheesh ...

And my speaker's rated input power is 700mW at an 8 ohm impedance (it's an SMS-1308MS-R from PUI)

So that's a 65.4 dB gap to bridge if I understand correctly ...

Okay, the LM386 can do that, and it's available in SMT. NO IT CANNOT, I MISREAD

I should check the output impedance of the SA605 and figure out if I need some sort of transformer, or perhaps to use a different resistor ... Is the output impedance set by the resistor if the SA605 truly has a current output? I can only assume the resistor is there because it's a current output...

That volume control pot will be hard to find in a form factor I don't despise. Almost makes me want to put a digital pot there, if one exists that can handle 1W.

RV09AF-20-20K-A15 ?
RV09AF-40-20K-A100K?
PTV09A-2020F-A104?

dear future self, you're not handling power across the speaker, but out of the mixer. It's minuscule. Stop trying to find a 1W audio taper pot

WHAT THE HELL WHY NOT
this is mouser's cheapest SPI digital POT. (I want to keep the i2c bus clear for other modules to control the radio as a whole)

Other, slightly less thought-out bits of the design

This is where I hash out bits of the design that I think are important, but I haven't devoted the brainpower or googlefu to yet. Most of the following will probably be more thought out after I have a mostly working schematic.

Test Points

In (the oft-referenced) this pdf, there was a test point for the second IF, and I liked the idea, so I kept it. I've added test points for the first IF, the first LO, the second LO, and may add both sides of the quad tank among other things. I assume a similar or higher resistor value than the one at the second IF will suffice in these places.

I once beheld a card from a cisco ONS fiber node which had the most delightful oscilloscope-attachable test points and I knew I must have them. They were nothing more than surface mountable tiny loops of metal, but they were perfect, and just small enough to be un-hobbyist-makeable.

Shielding

I have no idea how much is necessary or where, and haven't even thought about it. I may unmask bits of the ground plane throughout the board just in case I need to solder copper foil to it or whatever. ooh, or jam wires into through holes and wrap copper foil on that like a fence ...

Doing it properly

Of course, the proper way to do this would be with math and a thorough understanding of what's going on. I have a pretty good understanding of what's going on, but here's some resources anyway, perhaps to help me understand what I did if I actually make it work:

Alternate Design

Another possibility, besides a dual conversion superhet, is an HF receiver with a 2-meter down converter bolted on the front end. The difference here is where the VFO is - in the superhet, it's the first LO, in the HF plus down converter, it's the second. Big advantage here is that now the VFO is lower in frequency, and in DDSes that makes a huge difference in price. Just compare the AD9859 at $21.45 to the AD9833 at $9.79. Also, the 9833 can run at 5v, thus doesn't need another voltage regulator. And the 9833 is something I definitely can solder.

Anyway, part of my design goals in this radio is something that can be made cheaply, so I'm going to take a break and read the filter section of the ARRL Handbook and think about whether that's what I want to do.

Though lowering the frequency of the VFO sounds great, the IF following the VFO has to be high enough in frequency to allow for the tuning range desired (1/2 the range, to be precise). This is the whole reason I chose a double conversion design in the first place, because the SA605 is more stable at 455 KHz. That only allows for 910KHz tuning without running into an image frequency, and I need 4MHz.

So the basic upshot of this is that adding a fixed frequency downconverter up front doesn't allow for me to remove a stage. I still have to have a double conversion superhet after that. So, for now, I think I'll be focusing on the superhet without the downconverter in front.